The present invention relates to a curable composition for use in optical fiber cladding as well as to an optical fiber using such a cladding, and furthermore, in particular, relates to an optical fiber which has superior mechanical strength, environmental resistance, and optical characteristics, and to a curable composition for use in optical fiber cladding which can be applied to the manufacture thereof.
Plastic cladding optical fibers, hereinafter termed "PCF", in which the core is composed of quartz and the cladding is composed of plastic, have a comparatively low cost, superior transparency, and are capable of a high-level numerical aperture, so that they are used as light guides for optical fibers for medium distance transmission. Heretofore, silicon resin has been used as the cladding material for PCF; however, in order to improve its ease of handling and resistance to the environment, fluoride polymers having a high degree of hardness have recently been proposed and utilized as cladding material.
For example, in U.S Pat. Nos. 4,511,209; 4,707,076; Japanese Patent Application, First Publication Laid-Open No. Sho 63-40104; Japanese Patent Application, First Publication Laid-Open No. Sho 63-43104: Japanese Patent Application, First Publication Laid-Open No. Sho 63-208805; Japanese Patent Application, First Publication Laid-Open No. Sho 63-208806; Japanese Patent Application, First Publication Laid-Open No. Sho 63-208807; Japanese Patent Application, First Publication Laid-Open No. Sho 63-249112; European Patent No. 257863; and European Patent No. 333464; a composition for use in optical fiber cladding which is curable by activating energy rays and an optical fiber formed using this composition are described.
However, the curable composition for use in optical fiber cladding described in the above specifications has poor compatibility and homogeneity at room temperature, and when manufacture of optical fiber is undertaken at room temperature, the optical characteristics such as light transmission characteristics and the like of the optical fiber are extremely poor, and furthermore, the adhesion of the cladding layer to the core is inferior, so that separation of the cladding layer easily occurs, the resistance of the optical fiber to the environment and the tensile strength of the optical fiber are poor, and the resulting optical fiber is completely unfit for use. Furthermore, when, in order to increase the compatibility and homogeneity, the composition for use in the cladding is heated, strict control of the temperature is necessary in order to prevent problems such as eccentricity, and there are problems in that the wire drawing apparatus becomes complicated and operability becomes poor. In addition, in the conventional methods, when the composition for use in cladding is made transparent at room temperature, the mechanical strength of the cladding layer becomes inferior, the index of refraction rises, and it becomes impossible to maintain the numerical aperture which is desired.
Accordingly, the present situation is such that a composition for use in cladding which has superior operability with good transparency even at room temperature, has a low index of refraction, has superior transparency and mechanical strength even after hardening, and exhibits superior mechanical strength, optical characteristics, and resistance to the environment, such as resistance to heat and resistance to moisture, even in the case in which it is used in cladding for optical fibers, is not available.